Atomically thin heat shields could be up to 50,000 times thinner than current insulating materials in cell phones and laptops
Stanford University – August 19, 2019 --
Excess heat given off by smartphones, laptops and other electronic devices can
be annoying, but beyond that it contributes to malfunctions and, in extreme
cases, can even cause lithium batteries to explode.
To guard against such ills, engineers
often insert glass, plastic or even layers of air as insulation to prevent
heat-generating components like microprocessors from causing damage or
discomforting users.
Now, Stanford researchers have shown
that a few layers of atomically thin materials, stacked like sheets of paper
atop hot spots, can provide the same insulation as a sheet of glass 100 times
thicker. In the near term, thinner heat shields will enable engineers to make
electronic devices even more compact than those we have today, said Eric Pop,
professor of electrical engineering and senior author of a paper published Aug.
16 in Science Advances.
"We're looking at the heat in
electronic devices in an entirely new way," Pop said.
Detecting sound as heat
The heat we feel from smartphones or
laptops is actually an inaudible form of high-frequency sound.
If that seems
crazy, consider the underlying physics. Electricity flows through wires as a
stream of electrons. As these electrons move, they collide with the atoms of
the materials through which they pass. With each such collision an electron
causes an atom to vibrate, and the more current flows, the more collisions
occur, until electrons are beating on atoms like so many hammers on so many
bells -- except that this cacophony of vibrations moves through the solid
material at frequencies far above the threshold of hearing, generating energy
that we feel as heat.
Thinking about heat as a form of sound
inspired the Stanford researchers to borrow some principles from the physical
world. From his days as a radio DJ at Stanford's KZSU 90.1 FM, Pop knew that
music recording studios are quiet thanks to thick glass windows that block the
exterior sound. A similar principle applies to the heat shields in today's
electronics. If better insulation were their only concern, the researchers
could simply borrow the music studio principle and thicken their heat barriers.
But that would frustrate efforts to make electronics thinner. Their solution
was to borrow a trick from homeowners, who install multi-paned windows --
usually, layers of air between sheets of
glass with varying thickness -- to
make interiors warmer and quieter.
"We adapted that idea by creating
an insulator that used several layers of atomically thin materials instead of a
thick mass of glass," said postdoctoral scholar Sam Vaziri, the lead
author on the paper.
Atomically thin materials are a
relatively recent discovery. It was only 15 years ago that scientists were able
to isolate some materials into such thin layers. The first example discovered
was graphene, which is a single layer of carbon atoms and, ever since it was
found, scientists have been looking for, and experimenting with, other
sheet-like materials. The Stanford team used a layer of graphene and three
other sheet-like materials -- each three atoms thick -- to create a
four-layered insulator just 10 atoms deep. Despite its thinness, the insulator
is effective because the atomic heat vibrations are dampened and lose much of
their energy as they pass through each layer.
To make nanoscale heat shields practical,
the researchers will have to find some mass production technique to spray or
otherwise deposit atom-thin layers of materials onto electronic components
during manufacturing. But behind the immediate goal of developing thinner
insulators looms a larger ambition: Scientists hope to one day control the
vibrational energy inside materials the way they now control electricity and
light. As they come to understand the heat in solid objects as a form of sound,
a new field of phononics is emerging, a name taken from the Greek root word
behind telephone, phonograph and phonetics.
"As engineers, we know quite a lot
about how to control electricity, and we're getting better with light, but
we're just starting to understand how to manipulate the high-frequency sound
that manifests itself as heat at the atomic scale," Pop said.
Eric Pop is an affiliate of the Precourt
Institute for Energy. Stanford authors include former postdoctoral scholars
Eilam Yalon and Miguel Muñoz Rojo, and graduate students Connor McClellan,
Connor Bailey, Kirby Smithe, Alexander Gabourie, Victoria Chen, Sanchit
Deshmukh and Saurabh Suryavanshi. Other authors are from Theiss Research and
the National Institute of Standards and Technology.
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